LONDON (Reuters) - Deposed world number one Novak Djokovic suffered an early scare against Austrian debutant Dominic Thiem at the ATP World Tour Finals before launching his bid for a fifth successive title with a 6-7(10) 6-0 6-2 victory.

Thiem, the youngest of the eight qualifiers for the year-ender endeared himself to a sell-out O2 Arena crowd with a swashbuckling first set tiebreak which he sealed at the seventh time of asking with a sizzling forehand winner.

Djokovic, who could still end the year as world number one by doing better than new top dog Andy Murray, had looked reasonably comfortable, if a little flat, before Thiem upped the pace to take the opener.

Any chance of a shock opening loss to a man he had previously never dropped a set to in three meetings quickly subsided though as Djokovic found another gear to race through the second and third sets.

The second set was over in 23 minutes and once Djokovic forged a break ahead in the third game of the decider there was no way back for his Austrian opponent.

Later on Sunday in the Ivan Lendl group Gael Monfils, another first-timer at the event, takes on Canada's Milos Raonic.

Murray, who ended Djokovic's 122-week spell at the top of the rankings last week in Paris, faces Croatia's Marin Cilic in Monday's second John McEnroe group match after Japan's Kei Nishikori takes on U.S. Open champion Stan Wawrinka.

Home favourite Murray leads Djokovic by 405 points but with 275 Davis Cup final points about to drop off his total, the year-end positions will be decided in the coming days, possibly in a showdown in next Sunday's final.